The invention relates to selection of the serving network element in telecommunications systems, and particularly to advertising of mobile IP mobility agents or routers.
Mobility of employees has clearly increased during the past few years, and this trend is not expected to change. Attachment of a mobile node to the IP network involves, however, several problems. One solution to this problem is the mobile IP protocol defined by the mobile IP task force of the IETF (Internet Engineering Task Force). FIG. 1 illustrates a telecommunications system which supports the mobile IP protocol. The mobile IP refers to a mechanism for providing a mobile node user with telecommunications features using an IP address. It allows mobile nodes MN to change their Internet access point without changing their IP address. Thus the mobile IP facilitates traffic between the mobile node MN and a corresponding host CH, which communicates with the mobile node, through the home address of the mobile node MN.
In this application the term ‘mobile node’ MN refers to a host which wants to use the home network HN address when it is connected to a network other than the home network HN. This can be performed using e.g. telephone connections, ISDN connections, local network LAN connections (Local Area Network) or cellular connections, etc. The ‘home network’ HN may be a virtual IP network to which the user of the mobile node MN logically belongs. Its physical embodiment may be e.g. a local area network (LAN) which is connected to the Internet via a router. The ‘home address’ refers to an address assigned to the mobile node MN for rather a long period. It may remain unchanged regardless of where the mobile node MN connects to the Internet. A ‘home agent’ HA is a mobile IP mobility agent. The home agent is a routing entity which is located in the home network HN of the mobile node MN, transmits packets by tunnelling them for transmission to the mobile node MN when this is not in the home network HN, and maintains the current location information of the mobile node MN. Tunnelling means establishment of a virtual link, i.e. a tunnel TN, between nodes.
If the mobile node MN, when switched on or due to a transfer, notices that it is in a visited network VN, it can register with the home agent HA via another mobility agent, i.e. a foreign agent FA, and the mobile IP function can be activated. The foreign agent FA refers to a routing entity which is located in the visited network VN of the mobile node MN and offers routing services to the mobile node MN when it is registered, thus allowing the mobile node MN to use the address of its home network HN. The foreign agent FA transmits packets to the mobile node MN that have been tunnelled by its home agent HA. The foreign agent FA may also function as the default router of registered mobile nodes MN for the packets transmitted by the mobile node MN.
In the mobile IP protocol, a care-of address (COA) is defined as the end point of the tunnel for packets which are addressed to the mobile node MN in the visited network VN. The foreign agents FA may transmit advertising messages of mobile agents which include the COA to mobile nodes MN on their own initiative or at the request of the MN. One foreign agent can produce more than one COA in its advertising messages. The mobile node MN typically registers the COA received from the advertising message of the foreign agent FA in its home agent HA by sending a registration request. The home agent HA responds with a registration reply and maintains mobility binding for the mobile node MN. ‘Mobility binding’ means association of the home address with the COA, including the remaining lifetime of this association. The mobile node MN may have several COAs at the same time.
Depending on the IP network, a network may comprise several foreign agents which transmit advertising messages to mobile nodes MN. Thus the mobile node may receive advertising messages from several foreign agents. Furthermore, various foreign agents FA may have very different attributes, which may also vary dynamically according to the load situation, for example. The foreign agent and the COA to be used are typically selected by choosing the foreign agent FA that has sent the message received first, and thus selection of the foreign agent is by no means optimal.